Curable dental composites are widely used in dentistry to fill cavities. Dental composites with excellent tooth-like appearance can be formed on command with the advance of photopolymerization technology. The composites may contain a variety of materials and include monomers and a photoinitiator that generates initiating species (most commonly initiating radicals for majority of commercial dental composites) when exposed to a particular wavelength, thereby initiating polymerization of the monomers to cure the composite. Based on the mechanism by which initiating radicals are generated, photoinitiators for free radical polymerization are generally divided into two groups: 1) Norrish Type I photoinitiators, which undergo a unimolecular bond cleavage/dissociation upon irradiation to generate free radicals and 2) Norrish Type II photoinitiators, which undergo a bimolecular reaction where the excited state of the photoinitiator interacts with a co-initiator, forming excited state complex and to yield free radicals. There are many Norrish Type I and Type II photoinitiators for UV-curing applications, whereas relatively limited photoinitiators (mostly Type II) for visible light irradiation source.
However, visible blue light can also be scattered and absorbed by enamel and dentin, and certain down-conversion into longer wavelengths by way of fluorescence (such as 520 nm fluorescence emission by 410 nm excitation). As a dentist attempts to cure a dental composite by illuminating the tooth and composite from the top of the restoration and/or by directing the light from the side through dentin and enamel, much of the trans-tissue blue light is taken by way of attenuation (primarily light scattering, with certain absorption and fluorescence). As a result, this requires relatively high intensity (Irradiance) of visible blue light to penetrate through natural tooth structure.
Furthermore, clinical procedures using conventional composites have traditionally required building up the composite layer by layer. The incremental or layering placement is necessary due to polymerization shrinkage stress and depth of cure limitations. Restricted polymerization shrinkage, as one of the major drawbacks of dental composites, results in disrupting shrinkage stress at the interface between the composites and tooth, and can be transferred to the tooth structure.
Near infrared energy from about 800 nm to about 1200 nm (“Near-IR Therapeutic Window”) passes through natural dentition with little absorption and scattering, thus achieving significantly deeper penetration as compared to blue light (peak emission˜470 nm) radiation and was used in luminescent up-conversion of certain dental materials in Stepuk, A., et. al., “Use of NIR light and up conversion phosphors in light-curable polymers”, Dental Materials 28, (2012) 304-311. In this reference, the sodium salt of a yttrium fluoride host was co-doped with 25% ytterbium and 0.3% thulium (β-NaYF4:25%Yb3+, 0.3%Tm3+). The preparation was a solid salt that was then balled milled to particle diameters in the 2-3 micrometer range and incorporated into a dental adhesive (Heliobond).
Despite the use of a dental adhesive, the teachings of Stepuk are not transferable to the dental arts and there remain numerous voids not met by Stepuk. Among other unsatisfactory results, closer inspection of this reference reflects that upwards of 90 watts of 980 nm energy was applied to obtain 1 milliwatt of usable 490 nm radiation, which corresponds to an efficiency of approximately 0.001%. Accordingly, the teachings of Stepuk are not directly extendible to actual dental applications because the power required to achieve a useful result would cause an unacceptable temperature rise in the tooth pulp or other surrounding tissue. Stepuk also fails to teach any particle loading of greater than 20%, which would not even be sufficient to render it a dental composite and does not account for other constituents that might be included in the composite that further impact the effectiveness of the up-conversion.